The present invention relates to a synchronizing position detecting circuit that detects a received signal repeating a prescribed code to output a synchronizing position of the received signal and, in particular, relates to a synchronizing position detecting circuit suitable for a CDMA (Code Division Multiple Access) mobile station receiving device that carries out intermittent reception.
In the CDMA mobile communication system, for example, a base station multiplies an information signal by a spreading code and transmits it to a mobile station, while the mobile station demodulates the information signal by producing a spreading code having the same pattern and phase as those of the spreading code of the base station and multiplying the signal received from the base station by the produced spreading code (despreading), so that it is necessary for the mobile station to acquire phase information of the spreading code of the base station. On the other hand, depending on the mobile communication system, an intermittent reception system is employed wherein a mobile station performs reception processing only during a reception term and stops the reception processing during a waiting time (sleep time) for the purpose of reducing power consumption in the mobile station.
However, in the mobile station in the mobile communication system employing the intermittent reception system, since the reception processing is once stopped during the sleep time, it is impossible to track the phase of the spreading code of the base station during the sleep time, and thus it is difficult to immediately produce a spreading code in phase with the spreading code of the base station at the start of the next reception term. Therefore, for example, the mobile station, before starting the next reception term, receives a pilot signal transmitted from the base station, acquires a synchronizing position of the spreading code of the base station based on the phase information of the spreading code of the base station included in the pilot signal, and matches the phase of a spreading code produced by a spreading code generator with the phase of the spreading code of the base station based on the acquired synchronizing position, thereby to implement the normal reception processing from the start of the reception term.
In this case, in the mobile station, a synchronizing code generator produces a spreading code (synchronizing code) having the same pattern as that of the pilot signal for the purpose of detecting the pilot signal. However, since a slight error exists in the normal synchronizing code generator, the phase of the spreading code produced by the synchronizing code generator deviates from the correct phase with a lapse of the sleep time, for example. Thus, the pilot signal can not be captured after the lapse of the sleep time. Accordingly, the mobile station calculates a correlation function between the pilot signal and the synchronizing code while shifting the phase of the synchronizing code produced in the synchronizing code generator by a prescribed chip width within a prescribed phase range, and derives a position where the correlation function becomes maximum, thereby to detect the pilot signal and determine a synchronizing position of the pilot signal.
When detecting the pilot signal, it is desirable that the synchronizing code phase shifting range (window) be set as narrow as possible in terms of reduction in cost and power consumption of the receiving device. Therefore, taking into account that the magnitude of the phase shift of the synchronizing code due to the error of the synchronizing code generator is determined by the length of the sleep time, the window size is set depending on the period of the intermittent reception (e.g. see JP-A-H11-27180, pages 3-5 and FIG. 4).
In the foregoing pilot signal capturing circuit (synchronizing position detecting circuit), however, there has been a problem that since the window size is set only taking into account the phase shift of the synchronizing code based on the error of the synchronizing code generator, it can not deal with a phase shift of the synchronizing code caused by a change in distance between the base station and the mobile station due to movement of the mobile station during the sleep time, and thus there are those instances where the pilot signal can not be detected within the set window.